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RALPHK. BIRDWHISTELL University of West Florida Pensacola, FL 32504
Electrode
Outmoded Terminology: The R. W. Ramette Carleton College, Northfield, MN 55057
In reading textbooks and even papers in ACS journals I note that some authors report their electrode potentials "versus the NHE." This is incorrect because NHE means the "normal hydrogen electrode", which was used temporarily as a reference electrode in the early days of electrochemistry. Workers would immerse a platinum electrode into a solution of 1N strong acid and bubble hydrogen gas through the solution a t about 1 atm pressure. In this solution the activity of hydrogen ion is approximately 0.8 due to ionic interactions. The NHE terminology may be traced back to the classic article by Joel Hildebrand, "Some Applications of the Hydrogen Electrode in Analysis, Research and Teaching." l It may have originated with Nernst, long before there was an accepted distinction between concentration and activity. However, our internationally adopted electrode potential scale is based on the standard hydrogen electrode, SHE, a hypothetical electrode containing 1 m hydrogen ion having unit activity and no ionic interactions. Obviously this electrode cannot be made in the laboratory because finite concentration and ideal behavior are mutually exclusive. The (approximately) calculated difference in potential between the NHE and the SHE is quite significant. From the Nernst equation we see that for the half reaction 2H+(1 N, activity = 0.8)
+ 2e- = Ho(l atm)
with purified hydrogen gas bubbled through the solution a t whatever pressure is permitted by ambient conditions. The great importance of this practical electrode is that its potential can be accurately calculated from the Nernst equation, with the activity coefficient for hydrogen ion reliably estimated by the Davies equation to be 0.902 a t this low ionic strength. To illustrate, if the barometer reading is 745.0 mm of Hg for a hydrogen electrode at 25 OC with the vapor pressure of water a t 23.8 mm of Hg, we find for the calculated potential Ew = 0 - 29.6 log [(745.0 - 23.8)/760]/[0.01000 X 0.9021~ = -120.4 mV vs. the SHE
Such working electrodes are the practical primary standards for the determination of standard potentials for other couples and for the determination of pH and acid dissociation constants. For example, suppose we want to determine the standard potential of the silver couple by using the galvanic cell Pt/HCl(O.01000 m), H2(721.2mm of H~)/A~No.,'(~.o~ooo m)/Ag
The working hydrogen electrode on the left is the same as the one above that has a calculated potential of -120.4 mV vs. SHE. Suppose we measure the voltage of this cell a t 25 OC and find a value of +798.3 mV. By convention we have
ENHE= EO- 29.6 log (1atm)/(0.8)~- 5.7 mV
We still see the "normal" terminology, with respect to the various calomel electrodes for example. The "normal calomel electrode" made with 1 N (M) KC1 has a potential vs. SHE of +0.280 V, while the "tenth-normal" version has a value of +0.336 V. The "saturated" calomel electrode has a potential of +0.244 V. All are "real" electrodes whose potentials are measured directly versus a working hydrogen electrode (see below), unlike the hypothetical "standard calomel electrode" which has a potential of +0.268 V. I believe teachers and authors should make this significant distinction. "Normal", which refers to an actual concentration, should not be confused with "standard", which refers to unit activity and ideal behavior. The case is well made by Biegler and Woods2,who conclude that "the N.H.E. has neither fundamental nor operational significance and should be regarded as of historical interest only". What is of operational significance is the typical working hydrogen electrode, which comprises a P t electrode immersed in, say, 0.01000 m HC1 that is also in equilibrium
and from the Nernst equation we have = EOAg - 59.2 log [1/(0.01000 X 0.902)] - (-120.4)
whence the standard potential for the silver couple is +799.0 mV. This calculation has ignored the correction that could be applied to include the liquid junction potential between the two solutions. The NHE could not be used reliably for this determination because of the uncertainty in the activity coefficient of the hydrogen ion at such a high ionic strength. Thus, as educators we should not confuse the NHE with the SHE. We can still learn from Confucius: "If the terminology is not correct, then the whole style of one's speech falls out of form; a gentleman never uses his terminology indiscriminately." J. Am. Chem. Soc. 1913, 35,847-871. J. Chem. Educ. 1973, 50,604.
Volume 64
Number 10
October 1987
885
